This invention relates to a clamping apparatus and a method for clamping together the mold sections of an injection mold unit to form a sealed mold cavity and, more particularly, to a clamping apparatus and method for clamping an injection mold unit which is self-contained, uses only a small amount of energy, is compact in size, and is less expensive than standard mold clamps to purchase and operate.
Injection molding machines generally include a two-section mold unit wherein one of the mold sections is stationary and the other mold section generally is movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are sealed in contact engagement to form a mold cavity. The stationary mold section generally includes an opening for the injection of a mold material into the mold cavity. When the mold material is injected under high pressure into the mold cavity, the mold sections must be in positive sealed engagement to prevent the escape of mold material from the mold cavity. This sealed engagement is usually accomplished by clamping the mold sections together after the sections have been brought into engagement.
Clamping is typically accomplished with the use of very high pressure clamps which are secured to a mold frame. The mold frame generally has a set of tie bars interconnecting a pair of pressure plates. One pressure plate is typically fixed and nonmovable while the other pressure plate slides back and forth along the tie bars to allow the pressure plates to move into and out of proximity with one another. A high pressure clamp is braced against the mold frame on one end and secured to the movable pressure plate on the other. The entire assembly is designed to produce extreme amounts of pressure, between the movable pressure plate and the stationary pressure plate, with the high pressure clamp. The clamping assembly is typically of a general construction to allow various types of mold units to be placed within the clamping assembly. Before a mold unit can be placed between the pressure plates of a clamping assembly, a housing must be provided to allow the pressure plates to evenly contact the halves of the mold unit and to prevent uneven pressure being applied to the halves of the mold unit. Additionally, the mold unit typically requires an ejector assembly to remove the part after the molding process has been completed and a runner assembly to properly inject the plastic material into the mold cavity. These assemblies must also be provided with specially designed housings to allow them to fit properly between the mold units and the pressure plates and to allow these assemblies to withstand the extreme pressures generated during the molding process.
The requirement for the specially designed assemblies and housings leads to increased costs and added weight to the clamping apparatus. Although many such assemblies and housings are of a standard variety and, therefore, available without the added cost of customized tooling, very large or complicated parts typically require specialized housings, runners, and ejection assemblies, the manufacturing of which is time and capital intensive. Furthermore, since these specialized assemblies are generally integrated with the mold unit, failure of these assemblies leads to significant downtime. Not only must the failed assembly be replaced, but a new assembly must be integrated with the mold unit. Because withdrawing the failed assembly and integrating a new assembly can often not be done "in-house," a failed assembly can often take a mold unit out of service for several weeks.
The high pressure under which typical molding machines operate, coupled with the added weight of specialized housings requires a very large, very powerful clamping device to properly secure the mold units together. Additionally, due to the high pressure associated with such molding operations, the tie bars of the mold frame must be very strong, which also makes them unavoidably heavy. The cost of the parts associated with the clamping operation also increases with the weight and durability requirements of the parts. Therefore, typical molding machines are expensive as well as heavy.
The specialized housings for the mold unit, the runner assembly, and the ejection assembly also require an increased amount of daylight between the two pressure plates. This increased daylight requires that the clamping assembly be long to allow the mold units to retract enough from one another to allow the finished plastic article to be removed from the mold cavity. The added weight associated with this lengthening of the clamping assembly increases the overall weight of the apparatus.
Very large parts, such as bath tubs and freezer liners, require very large mold sections. Often, the size and expense of the clamping equipment required to maintain such large mold sections together throughout the molding process makes injection molding impractical for these applications. It would therefore be desirable to provide a feasible process for injection molding large parts.
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.